When does the crash sensor trigger?
When does the crash sensor trigger?
The crash sensor triggers when the measured vehicle deceleration exceeds a threshold of 3 to 5 g – equivalent to three to five times Earth's gravitational acceleration. For frontal collisions, the typical trigger threshold is a speed change of approximately 25–30 km/h.
What matters is not the driving speed, but how abruptly the vehicle is braked. A slow collision with a stationary car can already trigger the crash sensor, while a high-speed glancing blow against a guardrail may not be sufficient.
What conditions must be met?
For activation, at least two independent sensors must simultaneously detect an impact. Additionally, the safing sensor must confirm the collision. Only when this plausibility check is passed is the protection system activated.
Overview of trigger conditions:
- Deceleration: At least 3–5 g measured by acceleration sensors
- Dual confirmation: At least two sensors must agree
- Safing sensor: Additional safety instance must confirm
- Time window: The deceleration must occur within a few milliseconds
Difference: Speed vs. impact severity
The crash sensor does not measure driving speed, but rather deceleration during impact. A frontal collision at 30 km/h against a concrete wall triggers the sensor, while an impact at 80 km/h against a yielding guardrail may not be sufficient.
| Scenario | Speed | Triggered? | Reason |
|---|---|---|---|
| Frontal against concrete wall | 25–30 km/h | Yes | High deceleration, rigid obstacle |
| Glancing blow on guardrail | 80 km/h | Usually no | Low deceleration, soft obstacle |
| Rear-end collision into traffic jam | 50 km/h | Yes | High differential speed |
| Minor parking bump | 5 km/h | No | Threshold not reached |
Which protection systems are activated?
After the crash sensor triggers, the control unit activates various protection systems depending on accident severity: airbags, seat belt tensioners, seat belt force limiters, and in some vehicles also automatic door unlocking and emergency call.
- Seat belt tensioner: Tightens the belt and secures the occupant
- Airbags: Deployed in stages depending on impact severity
- Seat belt force limiter: Releases in a controlled manner after tensioning
- Roll bar: On convertibles for protection in rollover
- Door unlocking: Enables rescue personnel access
- eCall/Emergency call: Automatic emergency call in modern vehicles
How does triggering differ for frontal, side, and rear impacts?
Trigger criteria differ depending on impact direction. Side collisions require faster reaction since there is less crumple zone. Pressure sensors in the doors therefore react faster than acceleration sensors for frontal impacts.
| Impact type | Sensor type | Reaction time | Threshold |
|---|---|---|---|
| Frontal | Acceleration sensor | approx. 15–30 ms | 25–30 km/h differential |
| Side | Pressure sensor (door) | approx. 5–10 ms | Pressure rise in cavity |
| Rear | Acceleration sensor | approx. 15–30 ms | Similar to frontal |
What happens after triggering?
After triggering, the airbag control unit stores crash data and locks itself. The crash sensor itself usually remains intact but must be inspected together with the control unit. Deployed airbags and seat belt tensioners must always be replaced.
After an accident with sensor activation, the following steps are necessary:
- Do not continue driving – Airbag system is deactivated
- Visit a workshop – Diagnosis of the entire system
- Clear crash data – Reset data stored in the control unit
- Replace deployed components – Airbags, seat belt tensioners, possibly sensors
- Recalibrate system – Ensure full functionality
Frequently asked questions about crash sensor triggering
Here you will find answers to the most important questions about crash sensor triggering.
Can the crash sensor be triggered by a pothole?
Usually no. The plausibility check and safing sensor prevent false triggering from vertical impacts. The sensors are calibrated for horizontal deceleration during a collision.
Does the crash sensor trigger in a wildlife collision?
It depends on the size of the animal and the speed. In a collision with larger animals such as deer or wild boar at higher speeds, the trigger threshold can be reached.
Must the crash sensor be replaced after triggering?
Not necessarily. The crash sensor itself usually survives triggering undamaged. It only needs to be replaced if it is mechanically damaged or error codes are present. The airbag control unit, however, must always be serviced after stored crash data.
Why didn't my airbag deploy in an accident?
The deceleration was probably too low – for example, in an angled impact, a soft obstacle, or too low a speed change. The system is functioning correctly if the thresholds are not reached.